This invention relates to optical systems and, more particularly, to improved apparatus for de-emphasizing low spatial frequencies in optical imaging systems.
It is a characteristic of all optical pupils that low spatial frequencies are emphasized. This emphasis of low frequencies causes a derogation and often a complete masking of important optical information.
Various attempts have been made to eliminate this low spatial frequency bias. For example, in our paper Low Frequency De-Emphasis of the Modulation Transfer Function, I. One Dimensional Case, Optical Communications, Vol. 41, p. 388 (1982), N. Konforti and I disclosed an optical system in which an input image of an object is directed through a pair of pupils and an imaging lens. The lens is adjusted so that the object's image is projected onto an output image plane in the form of a photosensitive plate positioned behind a Ronchi ruling or grating. In this particular arrangement, the pupils are rectangular in shape and their centers are equidistant from the symmetry axis of the optical system. Each pupil provides an optical transfer function which is different from that of the other pupil. The two pupils are selected to have equal transmissivity, so that equal optical energy is transmitted through each pupil. One pupil is in the form of a single rectangle while the other pupil is formed of two rectangles.
The grating lines of the Ronchi grating are perpendicular to a line extending between the two pupils. The grating period and the distance between the grating and the photosensitive plate are chosen in accordance with geometric relationships specified in the paper. The images projected through the two pupils are sampled and interlaced by the Ronchi grating to form a single composite image on the photosensitive plate. This composite image is recorded on the photographic plate and, after developing, is analyzed with coherent illumination. The coherent light is diffracted by the recorded grating pattern and a first order beam is selected by an aperture properly located. The image formed by the beam associated with the first diffraction order is the desired output.
It is also postulated in the paper that one could generate a real-time version of the composite image by utilizing the photosensitive surface of a television camera in place of the photographic plate. The system disclosed in this paper is limited to the suppression of low spatial frequencies of the optical transfer function in only one dimension as a result of the rectangular pupils utilized. Another limitation of this sytem is the low level of illumination available at the output image plane due to the limited amount of light transmitted through the small pupils.
H. Bartelt and A. W. Lohmann, in their paper Optical Processing of One-Dimensional Signals, Optics Communications, Vol. 42, p. 87 (1982), discuss a general optical system for processing data which are not two-dimensional. The system utilizes two pupils in the form of single and double slits, respectively. A desired optical transfer function is synthesized by sequentially recording the results of two different filtering operations using a television camera and storing the data in an image memory. The two results are subtracted digitally and displayed on a monitor. Accordingly, while electronic sensing apparatus may be used as the photosensitive surface in systems of this type, a two step process is required to produce the result. As a consequence of the two-step process the output image is not available optically, thus preventing further optical processing or manipulation of the image. The amount of light reaching the output image plane is also severely limited.
C. Han and K. Murata, in their paper Two-Step Incoherent Optical Method for the Realization of a Rho Filter, Optics Letters, Vol. 8, p. 587 (1983), discuss an optical subtraction process in which individual circular statistical filters having a conventional low pass response with different cut-off frequencies are used to obtain a two dimensional rho filter by substraction of the optical transfer functions. Such a filter provides a linearly increasing transfer function for low spatial frequencies. Each of the two circular statistical amplitude filters has different sized opaque disks randomly distributed over its total aperture. The filters are placed, one after the other, in the system between an incoherently illuminated input image and a lens which forms an image on a vidicon receiving plane. Each image is separately digitized and stored in a digital image memory. By subtraction of the digitally stored images, a resultant image is obtained and displayed on the television monitor. This resultant image has the appropriate spatial frequencies de-emphasized. However, the process requires two steps to complete and, therefore, does not enable a real time solution. Moreover, the output image is not available optically, thus preventing further optical processing or manipulation of the image.
In our paper Low Frequency De-Emphasis of the Modulation Transfer Function, II. Two-Dimensional Case, Proceedings of the Tenth International Optical Computer Conference, p. 214, April, 1983, Boston, Mass., N. Konforti and I disclose a non-interacting circularly symmetric system utilizing two pupils, one circular and the other ring shaped, for de-emphasizing low spatial frequency components in two dimensions. The arrangement of the various system components is similar to that described above in the earlier Marom and Konforti paper. A Ronchi grating is employed to produce superposed interlacing images on a photosensitive material. Criteria are given for the design and placement of the Ronchi grating to obtain the appropriate composite image which may then be filtered to obtain the substractive result of the optical transfer functions of the two pupils. The criteria for selecting the dimensions of the pupils to obtain a band-pass filter characteristic are also provided. This filter system may be used to de-emphasize the low spatial frequencies and to allow a more correct presentation of optical information. This system also severely limits the amount of light reaching the output image plane.
It is one purpose of the present invention to provide an improved system for de-emphasizing low spatial frequencies in the optical analysis of incoherently illuminated two dimensional objects.
It is another purpose of the present invention to provide an improved system of low spatial frequency de-emphasis for incoherently illuminated two-dimensional objects which may be operated in real time and which transmits substantial amounts of light to the output image plane.
It is yet another purpose of the present invention to provide an optical image of an object with low spatial frequencies de-emphasized, whereby this image may be optically processed, analyzed and manipulated without the need of an electronic to optic converter.